Thomas Ridgeway, 1st Earl of Londonderry (1565? – 1631) was an English administrator active in Ireland, in particular in the Ulster Plantation.
Thomas Ridgeway, the 1st Earl of Londonderry, was son of Thomas Ridgeway of Tor Mohun, Devon, and Mary, daughter of Thomas Southcote of Bovey Tracey in the same county, and was born either at Torwood or at Tor Abbey about 1565. He matriculated at Exeter College, Oxford, on 17 November 1581, and was admitted a student of the Inner Temple in 1583. Subsequently he was collector of customs at Exmouth. He succeeded his father on 27 June 1597, and in July of that year fitted out a ship at his own cost to take part in the Islands Voyage under Robert Devereux, 2nd Earl of Essex. He was High Sheriff of Devon in 1600, and was knighted in the same year.
He is said to have taken part in the wars in Ireland, and may have done so under Lord Mountjoy. He was returned M.P. for Devon on 28 February 1604 to the Parliament of 1604–11, but resigned when appointed Treasurer of Ireland in 1606, a post which would require his long-term absence overseas. In 1603 he had been appointed vice-treasurer and treasurer-at-wars in Ireland under Lord Deputy Sir George Cary, whom he eventually succeeded as treasurer in April 1606. He held that office till 1616, being admitted a privy councillor on 20 October 1606. On 30 November 1606 he submitted a project to Robert Cecil, 1st Earl of Salisbury for increasing the crown revenues. On 18 December warrant was given to the Lord Chancellor of Ireland to issue a commission to him and certain others to inquire into abbey lands in County Dublin. He had apparently about this time been appointed master of the hawks and game in Ireland, an office formerly in the possession of Sir Geoffrey Fenton.